1. Field of the Invention
The present invention relates to a sheet feeding apparatus provided in an electrophotographic type or an electrostatic recording type image forming apparatus, such as a copying machine, a laser beam printer (LBP), or a facsimile apparatus, and adapted to feed sheets accommodated in a sheet feeding cassette successively to an image forming unit.
2. Description of the Related Art
Conventionally, in an image forming apparatus, such as a copying machine, a laser beam printer (LBP), or a facsimile apparatus, there is provided a sheet feeding apparatus that has a sheet feeding cassette detachable with respect to the apparatus main body and accommodating a sheet bundle consisting of a plurality of sheets, with the accommodated sheets being separated one by one and sent out to an image forming unit.
The sheet feeding apparatus is equipped with a sheet stacking plate supporting the sheet bundle within the sheet feeding cassette, and a lift unit raising the sheet stacking plate toward a feeding roller. The lift unit raises the sheet stacking plate by driving force of a drive source such as a motor, and presses an upper surface of a sheet supported by the sheet stacking plate against the feeding roller. In the sheet feeding apparatus, the feeding roller rotates, with the sheet and the feeding roller in press contact with each other, and the uppermost sheet is fed by frictional force of the feeding roller. As the lift unit, there is known a device having an operation unit raising the sheet stacking plate in the sheet feeding cassette after the sheet feeding cassette has been attached to the apparatus main body, and a lifter drive mechanism providing press contact force (hereinafter referred to as the sheet feeding pressure) to the sheet and the feeding roller via a pressurizing unit such as a spring. This technique is discussed in Japanese Patent Application Laid-Open No. 2006-56685.
However, in the sheet feeding apparatus equipped with the lift unit described above, when the lift unit raises the sheet stacking plate, the sheet feeding pressure may decrease due to sliding resistance between the sheet bundle on the sheet stacking plate and a side regulating member for regulating a side surface of the sheet bundle. This occurs because the side regulating member is maintained in a fixed state while the sheet stacking plate is rising.
This situation will be described with reference to a drawing. FIG. 10 illustrates a state in which a sheet bundle on a sheet stacking plate 31 is receiving sliding resistance from a side regulating member 32. When the sheet stacking plate 31 is lifted by the lift unit, it is done so with a sheet bundle end surface being rubbed against a regulating surface of the side regulating member 32. As indicated by arrows A in FIG. 10, in this state, the sheet bundle receives from the side regulating member 32 a force in a direction opposite to a direction of force (indicated by an arrow B) received from the sheet stacking plate 31. This force (indicated by the arrows A) constitutes resistance in lifting the sheet stacking plate 31, resulting in reduction in sheet feeding pressure. In this way, when the sheet feeding pressure is reduced, the sheet feeding roller cannot acquire requisite conveyance force, so that, in some cases, the sheet feeding apparatus fails to feed.
The larger a sheet size, the larger basic weight, and the larger a stacking amount, the more conspicuous is this tendency. The reduction in the sheet feeding pressure as described above is likely to be generated immediately after the lift-up operation to raise the sheet stacking plate. This is due to the fact that, while the sheets are being successively fed, vibration due to an apparatus operation such as a rotating operation of the sheet feeding roller is transmitted to the sheet stacking plate and the sheets, and due to the vibration, the rubbing of the sheet bundle end surface against the side regulating member as shown in FIG. 10 may be canceled.
FIG. 11 illustrates a relationship between the sheet stacking amount and the sheet feeding pressure when a predetermined pressure is provided to the sheet stacking plate 32. A line H1 indicates the relationship between a heavy paper sheet stacking amount and sheet feeding pressure, showing that, the larger the stacking amount, the greater the reduction in sheet feeding pressure. This is due to the fact that, when the sheet stacking amount increases, a rubbing area between the sheet bundle end surface and the side regulating member increases. A line H2 indicates the relationship between the sheet stacking amount and the sheet feeding pressure in the case where there is no reduction in the sheet feeding pressure due to sliding resistance. Similarly, a line L1 indicates the relationship between a plain paper sheet stacking amount and the sheet feeding pressure. A line L2 indicates the relationship between the sheet stacking amount and the sheet feeding pressure in the case where there is no reduction in the sheet feeding pressure due to sliding resistance. Comparison of the lines L1 and L2 shows that, although the reduction in the sheet feeding pressure becomes greater as the stacking amount increases, its influence is smaller as compared with the relationships of the lines H1 and H2 in the case of heavy paper sheets. As indicated by the line H1, when the reduction in the sheet feeding pressure becomes greater, a shortage of sheet feeding pressure occurs, and there is a fear of feeding failure occurring.
To cope with this problem, it might be possible to increase the pressure applied to the sheet stacking plate so as to compensate for the reduction in sheet feeding pressure, and to set the heavy paper sheet feeding pressure as indicated by a line H3. However, in the case of this setting, the plain paper sheet feeding pressure becomes higher than necessary as indicated by a line L3, and there is a fear of double feeding occurring.
In recent years, an image forming apparatus such as a copying machine, an LBP, or a facsimile apparatus, has been requested to be compatible with various sizes and types of sheets. Such sheets of various sizes and types include heavy sheets such as an ultra-heavy paper sheet in excess of 200 gf/m2, and a gloss coated paper sheet for high-quality color printing, and light sheets such as an ultra-light sheet, and a small size sheet of A6 size or card size. This means that, in FIG. 11, a difference in sheet feeding pressure between the line H1 which is the lower limit value for preventing feeding failure and the line L1 which is the upper limit value for preventing double feeding becomes greater. Thus, when the pressure applied to the sheet stacking plate is increased so as to compensate for the reduction in the sheet feeding pressure the heavy paper indicated by the line H1, the sheet feeding pressure of the plain paper indicated by the line L3 increases unnecessarily, so that the possibility of generation of double feeding increases. In this situation, for the image forming apparatus to be compatible with sheets of various types and sizes with a single pressurization unit, it is necessary to mitigate the reduction in the sheet feeding pressure due to the sliding resistance between the sheet bundle and the side regulating member.
In connection with this problem, there is known a configuration in which the pressure applied to the sheet stacking plate is switched according to a size and a type of a sheet. However, when such a configuration is adopted, it is necessary to additionally provide a detection unit for detecting the size and type of sheet, and to provide a mechanism for switching between a plurality of pressurization units, resulting in an increase in the number of components and an increase in cost. There is also known a system in which the pressure applied to the sheet stacking plate is switched according to the size and type of sheet. However, there is a fear of the user making an erroneous setting in this system. When sheets of different specifications from the setting are accommodated within the sheet feeding cassette, the sheet feeding pressure with respect to the sheet specifications is not optimum, and there is a fear of double feeding and feeding failure.
The above described problem of the reduction in the sheet feeding pressure is not restricted to the lift unit configuration as described with reference to the conventional example. A similar problem can arise in any configuration in which a sheet bundle on a sheet stacking plate is lifted while in sliding contact with a side regulating member regulating a side surface of the sheet bundle.